In this art, it has always previously been considered extremely difficult to combine high charge densities of propellant charges with the best possible burning properties of the charge. Charges have long been manufactured from bundled single or multi-perforated propellant sticks in full charge length, in which each propellant stick is as long as the entire charge and in which the sticks are packed together in parallel to form dense bundles. The ignition of such a charge presents no problem. On the contrary, such ignition is readily effected along the channels and outer sides of the propellant sticks as long as these are not coated with inhibitor. However, practically all of the propellant combustion will, in such a charge, take place in the cartridge chamber of the weapon, or in its immediate vicinity, which gives rise to extreme local wear on the barrel. Moreover, the pressure which the combustion gases give rise to within the long propellant tubes must be prevented from becoming so high that the propellant tubes, after a certain burn time, are split throughout their entire length and shattered into small fragments. In such an event, this gives rise to a relatively large instantaneous increase in the burning surface of the powder, which may result in a very high pressure elevation in the barrel which, in its turn, may naturally have disastrous effects upon the barrel itself. The holes or channels in the propellant tubes for such charges must, therefore, be made quite large, thus reducing the possibility of attaining high charge density and, in addition, reducing the progressivity of a multi-perforated propellant.
One attempt to avoid the effects briefly outlined above is disclosed in U.S. PS No. 660.567, dating from 1900, in which the inventor Gathmann proposes providing multi-hole, tubular propellant of long length with evenly spaced gas outlets in order to prevent the propellant tubes from being fragmented during the process of combustion. According to this patent, the gas outlets have been given the form of V-shaped broad grooves which are positioned alternately on opposing sides and extend at least past the center of the propellant tube. By such means the longitudinal channels of the propellant tube will be placed in communication with at least every second gas outlet. With this design, the inventor claims to be able to ensure that the propellant tubes will be kept whole throughout the combustion process and will not become fragmented. The inventor also claims to be able to achieve particular effects by varying the size of the grooves along the propellant rods. By such means, the inventor claims to be able to produce propellant charges which may be given uniform distribution throughout the volume of the charge chamber and which do not run the risk of becoming gathered together either at the front or at the rear, which could give rise to undesirable pressure peaks while the charge is burning.
However, we have long been aware of the fact that propellant charges consisting of loose tubular or rod-shaped propellant divided up into short lengths, so-called grain powder, most often impart to the charge the most highly advantageous burning properties and at the same time cause the least barrel wear. The reason for this is that loosely disposed powder in the propellant charge for cannons will, on combustion of the charge, in the main accompany the propellant gases and the projectile out into the barrel during successive combustion. This makes for considerably lower levels of local wear on the barrel in the critical zone immediately ahead of the charge chamber. At the same time short lengths of the propellant obviate the problems of fragmentation of the propellant tubes and consequential undesirable pressure peaks in the barrel. On the other hand, a desired pressure elevation in a charge of loosely disposed powder may be controlled, to a favorable point in time during the combustion process, by selecting single- or multi-perforated propellant of suitable hole diameter, possibly supplemented with a surface inhibition provided in a known manner. The disadvantage inherent in the loosely disposed grain powder is its considerable bulk and space requirement, since each grain of powder will then lie randomly oriented. Moreover, such loose powder charges require long ignitor tubes, or other types of igniting agents, extending along at least a portion of the charge and ensuring an instantaneous total ignition throughout a major part of the charge.
Otherwise, it is conceivable that the overall ignition of the charge will be uneven due to the high and uneven resistance to gas flow between the powder grains. In order to produce propellant charges of the same charge density as that which can be attained using bundled tubular propellant sticks of full charge length, but with the same burn properties as those which are attained in charges of loosely disposed tubular or rod-shaped granular propellant divided into smaller lengths, attempts have been made in this art to produce charges in which powder of the latter type has more or less manually been stacked, side by side, in layers one above the other. These charges have, functioned satisfactorily, but they are extremely expensive to produce manually and extremely difficult to produce by machine. Another method of increasing the performance of artillery pieces without recourse to a new design with room for larger propellant charges would then be to change to a propellant of higher force which, in its turn, automatically increases the level of wear on the barrel in a manner which is often unacceptable.
We have, however, now discovered that it is actually possible to produce propellant charges in which the propellant powder, on initiation, acts as a tubular propellant of large length in relation to the diameter of the combustion channel, quite simply because it then consists of such a propellant, but, after a brief interval in the continued combustion process, acts in the same manner as rod-shaped or tubular granular propellant divided up into short lengths, quite simply because it then consists of such granular propellant. The very fact that these charges may, moreover, be made with extremely high charge weights is a further advantage.
The solution to the problem has proved to be to form the charge of mutually parallel, tightly packed, single- or multi-hole tubular propellant rods, which, prior thereto, have been provided, at predetermined separations, with perforations from the outside of the propellant tubes to all of their longitudinal channels and preferably transversally through the propellant tube. These perforations may be effected either transversally through the center of the propellant tubes such that they cover all of the combustion channels of the propellant and leave a certain amount of propellant intact on either side of the perforations, which makes for the desired cohesion of the tubular propellant rods up to the instant of initiation, or alternatively, pairwise from opposite sides of the tubular propellant rods at a slight displacement from one another, such that they partially overlap. In the latter case each one covers but a part of the combustion channels of the propellant.
One essential difference in relation to the abovementioned U.S. patent is that we have found these perforations should be made without the removal of any material, such that the perforations will, at the moment of initiation, function as localized weak-points in the propellant tubes, rather than as gas outlets. The result will be that, because of the inner excess pressure of the combustion gases, the propellant tubes will, at a very early stage, become fragmented and thus form a grain propellant of a predetermined configuration. The weakening at each perforation must, therefore, be sufficiently large for the propellant tube to break completely at the perforations rather than become split along the propellant combustion channels. A suitable spacing between these perforations has been found to be between 10 and 100 times the inner diameter of the propellant tubes, i.e. the diameter of the combustion channels. Since each perforation should cover all longitudinal channels in the tubular propellant which may, for example, have 1-, 7-, 19- or 37-holes, or some other suitable number of channels, it is a distinct advantage to provide the perforations in such a manner that a sufficient amount of propellant is left on either side of the perforations in order that the propellant tue retain a sufficient inherent rigidity so as not to break up during both forming and handling of the charge. In propellant tubes of a length exceeding 100 times the diameter of their combustion channels, measures must be taken to ensure that the propellant tubes, on initiation, do not become fragmented in an uncontrolled manner. This problem may, in certain cases, occur even when powder tubes are of a length which is just above 10 times the diameter of the combustion channels. The propellant length which, in each individual case, gives rise to such uncontrollable combustion must thus be considered as excessive in this context. Thus, the term tubular propellant of considerable length in relation to the diameter of the combustion channels is here taken to mean lengths in excess of between 10 and 100 times the diameter of the combustion channels. One result of the dense packing of the propellant which we have succeeded in achieving in this way is that we have been able to pack wear-reducing "Swedish additive" in a modification of one of our older charges without needing, by compensation, to reduce weapon performance or increase the force of the propellant. On the contrary, the modified charge displays considerably better performance, whilst the wear-reducing additive has reduced barrel wear in a highly satisfactory manner.
The perforation of the tubular propellant rods may readily be executed in conjunction with the final shaping of the propellant by extrusion through a die. An automatic device for perforating the propellant tubes at predetermined separations can be provided in conjunction with the outlet side of the die, or elsewhere. In conjunction herewith, means for surface inhibition of the propellant tubes may be incorporated in those cases where it is desirable to produce a surfaceinhibited propellant with increased progressivity. Propellant charges according to the invention, wholly or partly consisting of surface-inhibited, progressive propellant are thus easy to produce. In this context, the present invention is highly relevant to this art, since a surface-inhibited propellant requires, as a rule, high charge rates in order to be fully effective. Charges of this type which have been subjected to tests have also proved to function highly satisfactorily. The surface inhibition may, depending upon the inhibitor, the coating method and the like, be effected either before or after the perforation.